1. Field of the Invention
The present invention relates to the field of dissolution testing and, in particular, to apparatuses for measuring heights in dissolution testing equipment.
2. Description of the Related Art
Dissolution testing and apparatuses for performing such are known in the art. U.S. Pat. Nos. 4,279,860 (Smolen) and 4,335,438 (Smolen) provide descriptions of the art of dissolution testing. In general, dissolution testing is used to determine the rate of dissolution of a material in a solution, such as dissolution testing of pharmaceuticals in solid, semi-solid, or transdermal dosage form. For example, dissolution testing may be used to determine the rate of dissolution of pharmaceuticals in dosage form in specific test solutions to simulate digestion in a human. The requirements for such dissolution testing apparatuses are provided in United States Pharmacopeia (USP), Edition XXII, Section 711, Dissolution (1990). A dissolution testing apparatus is described in U.S. Pat. No. 5,589,649, issued Dec. 31, 1996 to Brinker, the entirety of which is incorporated herein by reference.
Conventional dissolution testing devices have one or more test vessels in which test solutions may be placed. One conventional configuration of a dissolution testing apparatus has, for each test vessel, a basket-type stirring element consisting of a metal shaft with a cylindrical basket at the lower end thereof. After placing the dosage to be dissolved into the basket, the stirring element is lowered into the test solution near the center of the vessel and rotated at a specified rate (typically measured in revolutions per minute (RPM)) for a specified duration. Samples of the test solutions may be periodically withdrawn from the vessels to determine the degree of dissolution of the dosages as a function of time.
Another conventional configuration of a dissolution testing apparatus has a paddle-type stirring element consisting of a metal shaft with a metal blade at the end. In dissolution testing with this type of apparatus, the stirring element is rotated within the test solution with the dosage at the bottom of the vessel. In either conventional configuration, a vessel is utilized, along with a shaft having either a paddle or a basket at the bottom end thereof. In the present application, the term "stir element" may be used as a general term to refer either to a paddle or basket affixed to the bottom end of a shaft placeable within a dissolution testing vessel.
When inserting the shaft with stir element into the vessel, it is important to ensure that there is a precise, specified distance, or height, between the bottom of the stir element and the inside bottom of the vessel. In the present application, the distance between the bottom of the stir element and the inside bottom of the vessel may be referred to as the stir element height or simply height. This height is an important variable that can significantly affect the results of a dissolution test. Incorrect distances between paddle or basket and the bottom of the vessel can alter the fluid hydrodynamics and distort dissolution rates. The USP procedure for dissolution testing provides: "The distance of 25.+-.2 mm between the [paddle] blade [or basket] and the inside bottom of the vessel is maintained during the test."
Existing devices enable the setup of this distance in each vessel, but cannot quantify or verify the accuracy of this critical parameter. There is a need, therefore, for a means for ensuring that there is a precise, specified distance between the bottom of the stir element and the inside bottom of the vessel.
Further objects and advantages of this invention will become apparent from the detailed description of a preferred embodiment which follows.